Anion exchanger fillings through which flow can occur for electrolyte splitting in co2 electrolysis for better spatial distribution of gassing

ABSTRACT

An electrolysis cell having a multi-chamber structure, wherein an anion exchanger with a first ion exchanger membrane connects to a cathode chamber, wherein a salt bridge chamber connects to the first ion exchanger membrane, the salt bridge chamber with a fixed anion exchanger. An electrolysis system has such an electrolysis cell and a method for electrolysis of CO2 uses such an electrolysis cell or electrolysis system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2018/081741 filed 19 Nov. 2018, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 10 2017 223 521.7 filed 21 Dec. 2017. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention relates to an electrolysis cell with a multi-compartment structure, wherein a first ion exchange membrane comprising an anion exchanger is adjacent to a cathode compartment, wherein a salt bridge compartment which comprises a solid anion exchanger is adjacent to this first ion exchange membrane; an electrolysis system with such an electrolysis cell; and a method for the electrolysis of CO₂ using such an electrolysis cell or electrolysis system.

BACKGROUND OF INVENTION

At present, approximately 80% of world energy demand is met by the combustion of fossil fuels. Because of these combustion processes, approximately 34,032.7 million tons of carbon dioxide (CO₂) were released into the atmosphere worldwide in 2011. This release is the simplest way of disposing of even large amounts of CO₂ (with lignite-fired power plants accounting for over 50,000 t per day).

The discussion on the negative effects of the greenhouse gas CO₂ on climate has led to consideration of how to recycle CO₂. From a thermodynamic standpoint, CO₂ shows an extremely low value and therefore cannot readily be reduced to usable products.

In nature, CO₂ is converted by photosynthesis to carbohydrates. This process, which is divided into many partial steps both temporally, and on a molecular level, spatially, can be reproduced on an industrial scale only with great difficulty. The method that is currently more efficient compared to pure photocatalysis is the electrochemical reduction of CO₂. A mixed form is light-assisted electrolysis or electrically assisted photocatalysis. The two terms are to be used as synonyms, depending of the viewpoint of the observer.

In this process, as is the case in photosynthesis, CO₂ is converted to an energetically higher value product (such as CO, CH₄, C₂H₄, etc.) under supply of electrical energy (optionally photo-assisted), which can be obtained from renewable energy sources such as wind or sun. The amount of energy required in this reduction ideally corresponds to the combustion energy of the fuel and should only be derived from renewable sources. However, surplus production of renewable energy is not continually available, but at the moment only during times of strong solar radiation and heavy wind. However, this will be further increased in the near future with further development of renewable energy.

The electrochemical reduction of CO₂ on solid-state electrodes in aqueous electrolyte solutions provides a wide variety of product possibilities, with Faraday efficiencies on various metal cathodes being shown by way of example in Table 1, taken from “Electrochemical CO₂ reduction on metal electrodes” by Y. Hori, published in: C. Vayenas, et al. (Eds.)/Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189.

TABLE 1 Faraday efficiencies in the electrolysis of CO₂ on various electrode materials. Electrode CH₄ C₂H₄ C₂H₅OH C₃H₇OH CO HCOO⁻ H₂ Total Cu 33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 Ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0 102.0 Pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4 Hg 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9 3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6 100.1 Cd 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.0 0.0 95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0 0.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.0 0.0 0.0 0.0 0.0 99.7 99.7

The electrification of the chemical industry is currently being discussed. This means that preferably, chemical raw materials or fuels are intended to be produced from CO₂ (CO) and H₂O while preferably supplying surplus electrical energy from renewable sources. In the introductory phase of such a technology, efforts are being made to ensure that the economic value of a substance is significantly greater than its heating value (combustion value).

Electrolysis methods have undergone considerable further development over the past few decades. For example, it has been possible to optimize PEM water electrolysis with respect to high current densities. Large-scale electrolyzers showing performance in the megawatt range are being introduced onto the market.

It has been found in CO₂ electrolysis that coupling of the cathode to an anion exchange membrane (AEM) provides significant advantages with respect to selectivity, stability, and technical feasibility.

Moreover, it is known that by means of the AEM on the cathode, HCO₃ ⁻ produced as a byproduct can be further transported in the direction of the anode and decomposed into CO₂ at a site depending on the cell design, for example by protons formed on the anode side. It has been shown that it can be advantageous to use three-compartment cells in which the CO₂ is generated separately from the product of the electrodes, as this makes recycling easier. Corresponding cell designs can be found for example in US 2017037522 A1, DE 102017208610.6, and DE 102017211930.6.

In these cells, the cathode compartment is ordinarily delimited by an AEM. This allows cathodically produced anions such as HCO₃ ⁻, CO₃ ²⁻, and OH⁻ to be transported away in the direction of the anode. With respect to the configuration of the anode compartment and the gap between the anode and cathode compartments, these sources vary to a great degree. However, they have in common an area between the AEM, which delimits the cathode compartment, and the anode, said area containing a strongly acidic medium or producing protons, in which the HCO₃ ⁻ and CO₃ ²⁻ are decomposed by protonation into CO₂. Moreover, the charge transport in all of these cells can be carried in sections by various charge carriers. In contrast to other electrochemical arrangements, in this case the charge carriers are ordinarily not exchanged between the half cells, but are destroyed in the additional gap between them.

In US 2017037522 A1 and DE 102017211930.6, the central gap exclusively contains strongly acidic media. The generation of CO₂ therefore ordinarily takes place directly on the surface of the AEM, wherein in US 2017037522 A1, the medium of the gap is solid, while it is liquid in DE 102017211930.6. The surface of the AEM can be strongly impacted with gas bubbles, which can lead to a partial insulation of the membrane and thus to greater electrical losses in the cell. In addition, direct contact between strongly acidic solid media and the AEM should be avoided, as the solid media cannot avoid the CO₂ generated at this pH limit.

In DE 102017208610.6, the gap contains a neutral to weakly basic electrolyte, which as a rule contains carbonates. Therefore, the CO₂ generation ordinarily takes place on the surface of the second separator membrane, which can be just as problematic. In addition, it has been found that the use of salts, in particular with metal cations, in electrolyte gaps can be disadvantageous.

At present, there are no known solutions for the problems described above. The extent thereof, as described in DE 102017208610.6, can be reduced by increasing the system pressure. However, excessively increasing the pressure is not desirable because of the increased solubility of gases in water resulting therefrom.

There is thus a need for an improved electrolysis cell or an improved electrolysis system in which one can effectively prevent the binding of gas bubbles to a membrane in a multi-compartment system.

SUMMARY OF INVENTION

The inventors found that by using an additional electrolyzer component, it was possible in particular to improve cell voltage, operating stability, and energy efficiency. In this case, this component is preferably integrated such that in the resulting cell as a whole, neither salt encrustation of the electrodes nor CO₂ generation in the anode compartment are possible. The present invention thus constitutes a significant improvement over previously disclosed cell designs.

This is achieved in particular in that a salt bridge compartment in an electrolysis cell is filled with a solid anion exchanger that comprises, at least in the vicinity of the cathode/AEM, e.g. a hydrogencarbonate-, carbonate- and/or hydroxide-conductive, for example strongly basic anion exchanger. The anion exchanger makes it possible for gassing, for example the release of CO₂ in CO₂ electrolysis, to be distributed into the volume of the salt bridge compartment rather than taking place only at the AEM-salt bridge compartment interface.

In the following, the terms anion exchanger and anion transporter are used as synonyms. The transport function is characterized in that the anion exchange/anion transport material provides cations that compensate for the charge of the anions. According to certain embodiments, the anion itself is bound only so lightly that dynamic exchange is possible, thus providing a transport path for the anion in the electrolyte. At the same time, the cation is immobilized on the polymer backbone of the anion exchange material so that it cannot participate in charge transport processes.

In a first aspect, the present invention relates to an electrolysis cell, comprising —a cathode compartment comprising a cathode; —a first ion exchange membrane, which contains an anion exchanger and which is adjacent to the cathode compartment, wherein the cathode comes into contact with the first ion exchange membrane; —an anode compartment comprising an anode; and —a first separator, which is adjacent to the anode compartment; further comprising a salt bridge compartment, wherein the salt bridge compartment is arranged between the first ion exchange membrane and the first separator, wherein the salt bridge compartment comprises a solid anion exchanger, which is at least partially in contact with the first ion exchange membrane.

Further disclosed is an electrolysis system comprising an electrolysis cell according to the invention.

In addition, the present invention relates to a method for the electrolysis of CO₂, wherein an electrolysis cell according to the invention or an electrolysis system according to the invention is used, wherein CO₂ is reduced at the cathode and hydrogencarbonate and/or carbonate generated at the cathode migrates through the first ion exchange membrane to an electrolyte in the salt bridge compartment, wherein the hydrogencarbonate and/or carbonate is also transported through the solid anion exchanger in the salt bridge compartment away from the first ion exchange membrane.

Yet a further aspect of the present invention relates to the use of an electrolysis cell according to the invention or an electrolysis system according to the invention for the electrolysis of CO₂ and/or CO.

Further aspects of the present invention are to be found in the dependent claims and the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The attached drawings are intended to illustrate embodiments of the present invention and to facilitate further understanding thereof. In combination with the description, they serve to explain concepts and principles of the invention. Other embodiments and many of the advantages mentioned can be derived from the drawings.

The elements in the drawings are not necessarily shown to scale with respect to one another. Elements, features, and components that are identical, have the same function, or have the same action are indicated in the figures of the drawings by the same respective reference numbers unless otherwise specified.

FIGS. 1 to 9 are schematic diagrams of possible configurations of an electrolysis cell according to the invention.

In FIG. 10, a schematic diagram of an electrolysis system according to the invention is shown by way of example.

DETAILED DESCRIPTION OF THE INVENTION Definitions

Unless otherwise specified, the technical and scientific terms used herein have the same meaning as that which would be understood by the person skilled in the art in the field of the invention.

Quantities given in the context of the present invention refer to wt. % unless otherwise specified or unless the context clearly indicates otherwise. In the gas diffusion electrode according to the invention, the total of components in wt. % amounts to 100 wt. %.

In the context of the present invention, the term hydrophobic is understood to mean water-repellent. According to the invention, therefore, hydrophobic pores and/or channels are those which repel water. In particular, hydrophobic properties according to the invention are associated with substances or molecules having nonpolar groups.

In contrast, the term hydrophilic is understood to refer to the capacity to interact with water and other polar substances.

In general, gas diffusion electrodes (GDE) are electrodes in which liquid, solid and gaseous phases are present, and where in particular a conductive catalyst catalyzes an electrochemical reaction between the liquid and the gaseous phase.

The configuration can be of various types, for example a porous “solid material catalyst,” optionally with auxiliary layers for adjusting the hydrophobicity, wherein for example a membrane-GDE composite, e.g. an AEM-GDE composite, can then be produced; a conductive porous carrier to which a catalyst can be applied in a thin layer, wherein again a membrane-GDE composite, e.g. an AEM-GDE composite, can then likewise be produced; or a porous composite in the catalyst that can optionally be applied with an additive directly to a membrane, e.g. an AEM, and can then form in the composite a membrane coated with a catalyst (CCM; catalyst coated membrane).

The normal pressure is 101325 Pa=1.01325 bar.

Electro-osmosis: Electro-osmosis is understood to refer to an electrodynamic phenomenon in which a force is exerted toward the cathode on particles in solution with a positive zeta potential, and a force is exerted toward the anode on all particles having a negative zeta potential. If conversion occurs on the electrode, i.e. if a galvanic current flows, a material flow of the particles with a positive zeta potential to the cathode also takes place, regardless of whether the species is involved in the conversion or not. The same applies for a negative zeta potential and the anode. If the cathode is porous, the medium is also pumped through the electrode. This is also referred to as an electro-osmotic pump.

The material flows caused by electro-osmosis can also flow against the concentration gradients. In this manner, flows caused by diffusion that can offset the concentration gradients can be overcompensated for.

In a first aspect, the present invention relates to an electrolysis cell, comprising —a cathode compartment comprising a cathode; —a first ion exchange membrane, which contains an anion exchanger and which is adjacent to the cathode compartment, wherein the cathode comes into contact with the first ion exchange membrane; —an anode compartment comprising an anode; and —a first separator, which is adjacent to the anode compartment; further comprising a salt bridge compartment, wherein the salt bridge compartment is arranged between the first ion exchange membrane and the first separator, wherein the salt bridge compartment comprises a solid anion exchanger, which is at least partially in contact with the first ion exchange membrane.

The salt bridge compartment is not particularly limited, provided that it is correspondingly connected to the first ion exchange membrane at least partially, in particular mechanically or ionically, so that the solid anion exchanger can be at least partially in contact with the first ion exchange membrane therein. According to certain embodiments, the solid anion exchanger is in contact with the first ion exchange membrane at least essentially in an area in which the cathode is in contact with the first ion exchange membrane on an opposite side of this membrane, or in an area that is larger. This allows a favorable transfer of anions to be ensured that are generated in the cathode and fed through the first ion exchange membrane. Here, the term “in contact with the first ion exchange membrane” does not exclude the possibility that the contact does not take place over the entire surface, but is such, according to certain embodiments, that a material flow of fluids, i.e. liquids and/or gases, through the solid anion exchanger is also possible.

The term salt bridge compartment is used with respect to its function of acting as a “bridge” between the anode arrangement and cathode arrangement and comprising cations and anions which, however, in the present case do not have to form salts. In the present case, as at least one ion exchanger is present in the salt bridge compartment, one could also refer to said compartment as an ion bridge compartment. However, as this term is not commonly used, the compartment according to the invention will be referred to as a salt bridge compartment, even though it is not necessary for any salt to be present therein in the classical sense.

The dimensions of the salt bridge compartment are also not particularly limited, and it can be configured for example as a compartment or gap, e.g. between the first ion exchange membrane and the first separator, which for example are arranged parallel to each other.

The salt bridge compartment need not necessarily be in contact with the first separator that is adjacent to the anode compartment, i.e., more than three compartments may also be present in an electrolysis cell according to the present invention. The expression “arranged between the first ion exchange membrane and the first separator” thus means that the salt bridge compartment can be located at any desired position between the first ion exchange membrane and the first separator, provided that it comprises a solid anion exchanger that is at least partially in contact with the first ion exchange membrane. The salt bridge compartment is thus adjacent to the first ion exchange membrane, which however does not rule out the possibility that even a second separator or even further separators and/or further cell compartments are present and oriented toward the first separator. According to certain embodiments, the salt bridge compartment is in contact with the first separator. Therefore, the electrolysis cell according to the invention can be configured for example as a multi-compartment cell, e.g. a three-compartment cell, as described in US 2017037522 A1, DE 102017208610.6, and DE 102017211930.6, and reference is made thereto with respect to such cells. For example, therefore, a three-compartment cell may be present having three compartments (I, II, III). With the salt bridge compartment, electrolytic contact between the cathode compartment and the anode compartment can thus be achieved and/or facilitated.

The cathode compartment, anode compartment and salt bridge compartment are not particularly limited in the electrolysis cell according to the invention with respect to form, material, dimensions, etc., provided that they can accommodate the cathode, the anode and the first ion exchange membrane and the first separator. The three compartments are formed in the electrolysis cell according to the invention, wherein they can then be correspondingly separated, for example by the first ion exchange membrane and the first separator, for example with the first separator arranged between the salt bridge compartment and the anode compartment.

For the individual compartments, depending on the electrolysis to be carried out, inlet and outlet devices for reactants and products, for example in the form of a liquid, gas, solution, suspension, etc., can be correspondingly provided, wherein these can also optionally be recycled respectively. There is also no limitation in this respect, and flow can occur through the individual compartments in parallel flows or in counterflow. For example, in electrolysis of CO₂—which can further comprise CO, i.e. for example containing at least 20 vol. % CO₂—the CO₂ can be supplied to the cathode in solution, as a gas, etc.—there can for example be a counterflow to an electrolyte flow in the salt bridge compartment with a three-compartment configuration. There is no limitation in this respect.

There are corresponding possibilities for the inlet in the anode compartment as well, and these will also be discussed in further detail below. The respective inlet can be configured either continuously or discontinuously, for example in a pulsed configuration, etc. for which purpose corresponding pumps, valves, etc. can be provided in an electrolysis system according to the invention—which will also be further discussed below—as well as cooling and/or heating devices in order to allow corresponding catalysis of desired reactions at the anode and/or cathode.

The materials of the respective compartments or of the electrolysis cell and/or the further components of the electrolysis system can also be correspondingly adapted in a suitable manner to the desired reactions, reactants, products, electrolytes, etc. In addition, each electrolysis cell of course also comprises at least one power source. Further device components that occur in electrolysis cells or electrolysis systems can also be provided in the electrolysis system or electrolysis cell according to the invention. According to certain embodiments, these individual cells are combined into a stack that comprises 2-1000, preferably 2-200 cells, and the operating voltage of which is preferably in the range of 3-1500 V, particularly preferably 200-600 V.

According to certain embodiments, a gas formed in the salt bridge compartment, which e.g. corresponds to the reactant gas, e.g. CO₂, which may also optionally contain trace amounts of H₂ and/or CO, may be recycled back in the direction of the cathode compartment, where a corresponding return device may be provided in an electrolysis system according to the invention.

The cathode is not particularly limited according to the invention and can be adapted to a desired half reaction, for example with respect to the reaction products, provided that it is in direct contact with the first ion exchange membrane, i.e. is directly in contact with the first ion exchange membrane at at least one site, preferably wherein the cathode is essentially in direct planar contact with the first ion exchange membrane. The cathode is thus directly adjacent, at least in one area, to the first ion exchange membrane. A cathode for the reduction of CO₂ and optionally CO can for example comprise a metal such as Cu, Ag, Au, Zn, Pb, Sn, Bi, Pt, Pd, Ir, Os, Fe, Ni, Co, W, Mo, etc., or mixtures and/or alloys thereof, preferably Cu, Ag, Au, Zn, Pb, Sn, or mixtures and/or alloys thereof, and/or a salt thereof, wherein suitable materials can be adapted to a desired product. The catalyst can thus be selected according to the desired product. In the case of reduction of CO₂ to CO, for example, the catalyst is preferably based on Ag, Au, Zn and/or compounds thereof such as Ag₂O, AgO, Au₂O, AU₂O₃, ZnO. For the production of hydrocarbons, Cu or Cu-containing compounds such as CU₂O, CuO and/or copper-containing mixed oxides with other metals, etc., are preferred. For a production of formic acid, for example, catalysts based on Pb, Sn and/or Cu, in particular Pb, Sn, may be used. As according to certain embodiments, hydrogen formation at high current densities may be completely inhibited by anion transport, catalysts for CO₂ reduction that do not possess high overvoltage with respect to hydrogen can be used, e.g. reduction catalysts such as Pt, Pd, Ir, Os or carbonyl-forming metals such as Fe, Ni, Co, W, Mo. Thus the described operating method, in combination with the cell design, opens up new pathways in CO₂ reduction chemistry that are not dependent on hydrogen overvoltage.

The cathode is the electrode on which the reductive half reaction takes place. It can have a single or multiple component(s) and can be configured for example as a gas diffusion electrode, a porous electrode or directly with the AEM in the composite, etc.

The following embodiments are possible, for example: —a gas diffusion electrode or porous bound catalyst structure, which according to certain embodiments can e.g. be ion-conductively and/or mechanically bonded by means of a suitable ionomer, for example a anionic ionomer, to the first ion exchange membrane, for example an anion exchange membrane (AEM); —a gas diffusion electrode or porous bound catalyst structure, which according to certain embodiments can be partially pressed onto the first ion exchange membrane, for example an AEM; —a porous, conductive, catalytically inactive structure, e.g. a carbon-paper GDL (gas diffusion layer), a carbon-cloth GDL, and/or a polymer-bound film of granular vitreous carbon, which is impregnated with the catalyst of the cathode and optionally an ionomer that allows the binding to the first ion exchange membrane, for example an AEM, wherein the electrode can then be mechanically pressed onto the first ion exchange membrane, for example an AEM, or can be pre-pressed together with the first ion exchange membrane, for example an AEM, in order to form a composite; —a particulate catalyst, which is applied by means of a suitable ionomer to a suitable carrier, for example a porous conductive carrier, and according to certain embodiments can be adjacent to the first ion exchange membrane, for example an AEM; —a particulate catalyst, which is pressed into the first ion exchange membrane, for example an AEM, or is coated thereon and for example is correspondingly conductively bound, wherein this structure can then be pressed for example as a so-called CCM (catalyst-coated membrane) onto a conductive, porous electrode, wherein a catalytic activity of this electrode is generally not necessary and for example carbon-based GDLs or gratings, for example of titanium, can be used, wherein it is not excluded for this electrode to contain ionomers and/or the active catalyst or to consist in large part thereof; —a non-closed flat structure, e.g. a mesh or a metal mesh, which for example consists of or comprises a catalyst or is coated therewith and according to certain embodiments is adjacent to the first ion exchange membrane, for example an AEM; —a polymer-bound solid catalyst structure of a particulate catalyst, which comprises an ionomer that allows binding to the first ion exchange membrane, for example an AEM, or has been subsequently impregnated therewith, wherein the electrode is then mechanically pressed onto the first ion exchange membrane, for example an AEM, or can be pre-pressed together with the first ion exchange membrane, for example an AEM, in order to form a composite; —a porous, conductive carrier, which is impregnated with a suitable catalyst and optionally an ionomer and according to certain embodiments is adjacent to the first ion exchange membrane, for example an AEM; —a non-ion-conductive gas diffusion electrode, which has subsequently been impregnated with a suitable Ionomer, for example an anion-conductive Ionomer, and according to certain embodiments is adjacent to the first ion exchange membrane, for example an AEM, or is bound thereto, e.g. via an ionomer.

Different combinations of the above-described electrode structures are also possible for use as a cathode.

The corresponding cathodes can also contain materials commonly used in cathodes, such as binders, ionomers, for example anion-conductive ionomers, fillers, hydrophilic additives, etc., which are not particularly limited. In addition to the catalyst, the cathode can also, according to certain embodiments, comprise at least one ionomer, for example an anion-conductive or anion-transporting ionomer (e.g. an anion exchange resin, an anion transport resin) which e.g. can comprise various functional groups for ion exchange that can be the same or different, for example tertiary amine groups, alkylammonium groups and/or phosphonium groups), an e.g. conductive carrier material (e.g. a metal such as titanium), and/or at least one non-metal such as carbon, Si, boron nitride (BN), boron-doped diamond, etc., and/or at least one conductive oxide such as indium tin oxide (ITO), aluminum zinc oxide (AZO) or fluorinated tin oxide (FTO)—for example as is used for the production of photoelectrodes, and/or at least one polymer based on polyacetylene, polyethoxythiophene, polyaniline or polypyrrole, such as for example in polymer-based electrodes; non-conductive carriers such as e.g. polymer networks are possible for example if the catalyst layer has sufficient conductivity), binders (e.g. hydrophilic and/or hydrophobic polymers, e.g. organic binders, e.g. selected from PTFE (polytetrafluorethylene), PVDF (polyvinylidene difluoride), PFA (perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene copolymers), PFSA (perfluorosulfonic acid polymers), and mixtures thereof, in particular PTFE), conductive fillers (e.g. carbon), non-conductive fillers (e.g. glass) and/or hydrophilic additives (e.g. Al₂O₃, MgO₂, hydrophilic materials such as polysulfone, e.g. polyphenylsulfone, polyimide, polybenzoxazole or polyether ketone or polymers that are generally electrochemically stable in the electrolyte, polymerized “ionic liquids,” and/or organic conductors such as PEDOT:PSS or PANI (camphor sulfonic acid doped polyaniline), which are not particularly limited.

The cathode, in particular in the form of a gas diffusion electrode, e.g. connected to the first ion exchange membrane, or contained in the form of a CCM, comprises according to certain embodiments ion-conductive components, in particular an anion-conductive component.

Other cathode forms are also possible, for example cathode structures such as those described in US 20160251755 A1 and U.S. Pat. No. 9,481,939.

The anode is also not particularly limited according to the invention and can be adapted to a desired half reaction, for example with respect to the reaction products. The oxidation of a substance takes place in the anode compartment on the anode, which is electrically connected to the cathode by means of a power source for supplying the voltage for the electrolysis. In addition, the material of the anode is not particularly limited and depends primarily on the desired reaction. Examples of anode materials include platinum or platinum alloys, palladium or palladium alloys and vitreous carbon, iron, nickel, etc. Further anode materials are also conductive oxides such as doped or undoped TiO₂, indium tin oxide (ITO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), iridium oxide, etc. Optionally, these catalytically active compounds can also be superficially applied only in thin-film technology, for example to a titanium and/or carbon carrier. The anode catalyst is not particularly limited. For example, as a catalyst for O₂ or Cl₂ production, one also uses IrO_(x) (1.5<x<2) or RuO₂. These can also be present as mixed oxides with other metals, e.g. TiO₂, and/or be supported on a conductive material such as C (in the form of conductive carbon black, activated carbon, graphite, etc.). Alternatively, catalysts based on Fe—Ni or Co—Ni can also be used for O₂ generation. For example, the structure described below with a bipolar membrane or a bipolar membrane is suitable for this purpose.

The anode is the electrode on which the oxidative half reaction takes place. It can also be configured as a gas diffusion electrode, a porous electrode, or a full electrode or solid electrode, etc.

The following embodiments are possible: —a gas diffusion electrode or porous bound catalyst structure, which according to certain embodiments can be e.g. ion-conductively and/or mechanically bonded by means of a suitable ionomer, for example a cationic ionomer, to the first separator, for example a cation exchange membrane (CEM) or a diaphragm; —a gas diffusion electrode or porous bound catalyst structure, which according to certain embodiments can be partially pressed into the first separator, for example a CEM or a diaphragm; —a particulate catalyst, which is applied by means of a suitable ionomer onto a suitable carrier, for example a porous conductive carrier, and according to certain embodiments can be adjacent to the first separator, for example a CEM or a diaphragm; —a particulate catalyst, which is pressed into the first separator, for example a CEM or a diaphragm, and for example is correspondingly conductively bound; —a non-closed flat structure, e.g. a mesh or a metal mesh, which for example consists of or comprises a catalyst or is coated therewith and according to certain embodiments is adjacent to the first separator, for example a CEM or a diaphragm; —a solid electrode, wherein in this case there can also be a gap between the first separator, for example a CEM or a diaphragm, and the anode; —a porous, conductive carrier, which is impregnated with a suitable catalyst and optionally an ionomer and according to certain embodiments is adjacent to the first separator, for example a CEM or a diaphragm; —a non-ion-conductive gas diffusion electrode, which has been subsequently impregnated with a suitable Ionomer, for example a cation-conductive ionomer, and according to certain embodiments is adjacent to the first separator, for example a CEM or a diaphragm; —any desired variants of the discussed embodiments, wherein the electrode e.g. contains an anodically stable anion-conductive material and is directly adjacent to the anion-conductive layer of a bipolar membrane.

In this case as well, various combinations of the different anode structures are possible.

The corresponding anodes can also contain materials commonly used in anodes, such as binders, ionomers, e.g. also cation-conductive ionomers, for example containing sulfonic acid and/or phosphonic acid groups, fillers, hydrophilic additives, etc., which are not particularly limited, and which for example are also described above with respect to the cathode.

According to certain embodiments, the cathode and/or the anode is/are configured as a gas diffusion electrode, as a porous bound catalyst structure, as a particulate catalyst on a carrier, as a coating of a particulate catalyst on the first and/or second ion exchange membrane, as a porous conductive carrier in which a catalyst is impregnated, and/or as a non-closed flat structure. According to certain embodiments, the cathode is configured as a gas diffusion electrode, as a porous bound catalyst structure, as a particulate catalyst on a carrier, as a coating of a particulate catalyst on the first and/or second ion exchange membrane, as a porous conductive carrier in which a catalyst is impregnated, and/or as a non-closed flat structure, which contain(s) an anion exchange material and/or an anion transport material. According to certain embodiments, the anode is configured as a gas diffusion electrode, as a porous bound catalyst structure, as a particulate catalyst on a carrier, as a coating of a particulate catalyst on the first and/or second ion exchange membrane, as a porous conductive carrier in which a catalyst is impregnated, and/or as a non-closed flat structure, which contain(s) a cation exchange material and/or is/are coupled and/or bound to a bipolar membrane.

According to certain embodiments, the anode and/or the cathode is/are brought into contact on the side opposite the salt bridge compartment with a conductive structure. The conductive structure is not particularly limited here. According to certain embodiments, the anode and/or the cathode are thus brought into contact with side facing away from the salt bridge with conductive structures. These are not particularly limited. For example here, these can be carbon flows, metal foams, metal knit fabrics, metal meshes, graphite structures, or metal structures.

In an electrolysis cell according to the invention as well as in the methods according to the invention, the above electrodes mentioned by way of example can be combined with one another in any desired manner.

In addition, electrolytes can also be present in the anode compartment and/or cathode compartment, and these are also referred to as the anolyte or catholyte, but it is not excluded according to the invention for no electrolytes to be present in the two compartments, and correspondingly, for example, for only gases to be fed to said compartments for conversion, for example only CO₂, optionally also as a mixture with e.g. CO and/or H₂O, which can optionally also be a fluid, e.g. an aerosol, but preferably gaseous H₂O to the cathode and/or water or HCl to the anode. According to certain embodiments, an anolyte is present, which can be different from an electrolyte of the salt bridge compartment or can correspond thereto, for example with respect to solvents, acids etc. contained therein.

Here, a catholyte is the electrolyte flow around the cathode or on the cathode and serves according to certain embodiments to provide the cathode with substrate or reactant.

For example, the following embodiments are possible. The catholyte can be present e.g. as a solution of substrate (CO₂) in a liquid carrier phase (e.g. water) and/or as a mixture of the substrate with other gases (e.g. CO+CO₂; water vapor+CO₂, N₂ and/or also certain proportions of O₂, SO₂, SO₃; etc.). Gases recycled through a return line, such as CO and/or H₂, can also be present. The substrate can also be in the form of a pure phase, e.g. CO₂. If uncharged liquid products are generated in the reaction, they can be washed out by the catholyte and can then optionally also be correspondingly separated out.

An anolyte is an electrolyte flow around the anode or at the anode and serves according to certain embodiments to provide the anode with substrate or reactant, and optionally to transport anode products away. For example, the following embodiments are possible. The anolyte can be present as a solution of the substrate (e.g. sulfuric acid=HCl_(aq)) in a liquid carrier phase (e.g. water), optionally with conductive salts, which are not limited—in particular in the use of a bipolar membrane as a first separator membrane, wherein the anolyte can also become basic and can also contain cations, as described below, or as a mixture of the substrate with other gases (e.g. hydrogen chloride=HCl_(g)+H₂O, SO₂, etc.). As is also the case for the catholyte, however, the substrate can also be in the form of a pure phase, e.g. in the form of hydrogen chloride gas=HCl_(g).

According to certain embodiments, the anolyte is an aqueous electrolyte, wherein corresponding reactants, which are converted at the anode, can optionally be added to the anolyte. Addition of the reactant in this case is not particularly limited. Moreover, reactant addition at the inlet to the cathode compartment is also not limited. For example, CO₂ can thus be added outside the cathode compartment to water, or can also be added via a gas diffusion electrode, or can also be supplied only as a gas to the cathode compartment. Analogously, corresponding configurations are possible for the anode compartment, depending on the reactant used, e.g. water, HCl, NaCl, KCl, etc. and the desired product.

The first ion exchange membrane, which contains an anion exchanger and/or anion transporter or an anion transport material and which is adjacent to the cathode compartment, is not particularly limited according to the invention. In the electrolysis cell according to the invention and the method according to the invention, it separates the cathode from the salt bridge compartment, such that the sequence from the direction of the cathode compartment in the direction of the electrolyte is cathode/first ion exchange membrane/salt bridge compartment. In particular, according to certain embodiments, it contains an anion exchanger or is composed thereof, which is present in the currentless state in the form of an acid-anion salt, preferably corresponding to an acid that is present in the salt bridge compartment, and is further preferably converted to the hydrogencarbonate/carbonate form as of a minimum current density. According to certain embodiments, the first ion exchange membrane is an anion exchange membrane and/or an anion transport membrane. According to certain embodiments, the first ion exchange membrane can comprise a hydrophobic layer, for example on the cathode side, for better gas contact. Preferably, the ion exchange membrane and/or anion transport membrane also functions as a cation blocker (even if only in trace amounts, for example), in particular a proton blocker. In particular, an anion exchanger and/or anion transporter with solidly bound cations can constitute a blockade here for mobile cations through Coulomb repulsion, which can additionally counteract salt precipitation, in particular within the cathode.

In the case of a membrane-electrode arrangement (MEA) in particular, the accumulation of the electrolyte cations in the area of the interface is ordinarily attributable to electro-osmosis. In this case, it can be difficult to reduce a concentration gradient on the electrode side, as a catalyst-based cathode that is configured as discussed above, e.g. a gas diffusion electrode or a CCM, may show only extremely poor anion conductivity depending on the anion and a selected electrolyte. In this case, the anion conductivity can be significantly improved by the integration of anion-conductive components.

In order to improve operating stability, ion transporters, in particular anion transport resins, can be used as a binding material or an additive in the electrode itself and/or in an anion exchange layer adjacent to the cathode, in order for example to quickly dissipate or partially buffer any Off ions generated, such that the reaction with CO₂ and the accompanying formation of hydrogencarbonates and/or carbonates can be reduced or the anion transport resins themselves conduct HCO₂ or CO₃ ²⁻. In principle, anion transport can take place by means of anion exchangers. Moreover, an integrated anion exchanger in particular also constitutes a blockade for cations, e.g. metal cation traces as well, which can additionally counteract salt precipitation and contamination of the electrode. In the case of protons, for example, hydrogen formation can also be inhibited.

The first ion exchange membrane can thus for example contain an anion exchanger and/or an anion transporter in the form of an anion exchange and/or transport layer, wherein further layers can then be included, such as hydrophobicity-imparting layers in order to improve the contact with a gas such as CO₂. According to certain embodiments, the first ion exchange membrane is an anion exchange membrane and/or anion transport membrane, e.g. for example an ion-conductive membrane (or also in the broader sense a membrane with an anion exchange layer and/or an anion transport layer) with positively charged functional groups, with this not being particularly limited. A preferred charge transport takes place in the anion exchange layer and/or the anion transport layer or an ion exchange membrane and/or an anion transport membrane by means of anions. In particular, the first ion exchange membrane and/or in particular an anion exchange layer and/or an anion transport layer or an anion exchange membrane and/or an anion transport membrane therein serve(s) to provide an anion transport along stationary fixed positive charges. In this manner, in particular, penetration of an e.g. proton-containing electrolyte into the cathode due to electro-osmotic forces can be reduced or completely prevented. In particular, according to certain embodiments, the ion exchanger contained in the membrane can be converted in operation to the carbonate/hydrogencarbonate form and the passage of protons through the membrane to the cathode can thus be prevented.

According to certain embodiments, a suitable first ion exchange membrane, for example an anion exchange membrane and/or an anion transport membrane, shows favorable wettability by water and/or acids, in particular aqueous acids, high ion conductivity, and/or a tolerance of the functional groups contained therein with respect to high pH values, and in particular shows no Hoffmann elimination. An exemplary AEM according to the invention is the A201-CE membrane produced by Tokuyama used in the example, the “Sustainion” produced by Dioxide Materials, or an anion exchange membrane produced by Fumatech such as e.g. the Fumasep FAS-PET or the Fumasep FAD-PET.

Otherwise, the first separator is not particularly limited.

According to certain embodiments, the first separator, which for example according to certain embodiments is adjacent to the salt bridge compartment seen from the anode side, is selected from an ion exchange membrane containing a cation exchanger, a bipolar membrane, wherein preferably, in said bipolar membrane, the cation-conductive layer is oriented toward the cathode and the anion-conductive layer is oriented toward the anode, and a diaphragm. According to certain embodiments, the first separator is a cation exchange membrane, a bipolar membrane or a diaphragm.

A suitable first separator, for example a cation exchange membrane or a bipolar membrane, contains for example a cation exchanger, which can be in contact with the salt bridge compartment. For example, it can contain a cation exchanger in the form of a cation exchange layer, wherein further layers such as hydrophobicity-imparting layers can then be included. It can also be configured as a bipolar membrane or as a cation exchange membrane (CEM). The cation exchange membrane or cation exchange layer is e.g. an ion-conductive membrane or ion-conductive layer with negatively charged functional groups. An exemplary charge transport into the salt bridge compartment takes place in such a first separator by means of cations. For example, commercially available Nafion® membranes are suitable as a CEM, or also the Fumapem-F membranes produced by Fumatech, Aciplex produced by Asahi Kasei, or the Flemion membranes produced by AGCs. In principle, however, other modified polymer membranes with strongly acidic groups (groups such as sulfonic acid or phosphonic acid) can also be used. According to certain embodiments, the first separator prevents the movement of anions, in particular HCO₃ ⁻, into the anode compartment.

In addition, in the electrolysis cell according to the invention, as well as in the method according to the invention, the first separator can be configured as a diaphragm, which allows the cell to be configured in a less complex and expensive manner. According to certain embodiments, the diaphragm essentially separates the anode compartment and the salt bridge compartment, for example to more than 70%, 80%, or 90%, based on the interface between the anode compartment and the salt bridge compartment, or separates the anode compartment and the salt bridge compartment, i.e. to 100%, based on the interface between the anode compartment and the salt bridge compartment. The same also applies to other first separators. Particularly preferred are embodiments that produce gas separation, e.g. of the CO₂ in the salt bridge compartment and the O₂ in the anode compartment.

The diaphragm is not particularly limited and can for example be based on a ceramic (e.g. ZrO₂ or Zr₃(PO₄)₃) and/or a swellable functionalized polymer, e.g. PTFE. Binders (e.g. hydrophilic and/or hydrophobic polymers, e.g. organic binders, e.g. selected from PTFE (polytetrafluorethylene), PVDF (polyvinylidene difluoride), PFA (perfluoroalkoxy polymers), FEP (fluorinated ethylene-propylene copolymers), PFSA (perfluorosulfonic acid polymers), and mixtures thereof, in particular PTFE)), conductive fillers (e.g. carbon), non-conductive fillers (e.g. glass) and/or hydrophilic additives (e.g. Al₂O₃, MgO₂, hydrophilic materials such as polysulfone, e.g. polyphenylsulfone (PPSU), polyimide, polybenzoxazole or polyether ketone or polymers that are generally electrochemically stable in the electrolyte can also be present.

According to certain embodiments, the diaphragm is porous and/or hydrophilic. As it is not ion-conductive per se, it should preferably be capable of swelling in an electrolyte, for example an acid. Moreover, it constitutes a physical barrier for gases and cannot be penetrated by gas bubbles. For example, it is a porous polymer structure, wherein the base polymer is hydrophilic (e.g. PPSU). In contrast to the CEM or bipolar membrane, the polymer does not comprise any charged functional groups. Further preferably, moreover, the diaphragm can contain hydrophilic structuring components such as metal oxides (e.g. ZrO₂ and/or other materials such as particles over the surface thereof) or ceramics, as mentioned above.

According to certain embodiments, a suitable first separator, for example a cation exchange membrane, a bipolar membrane and/or a diaphragm, shows favorable wettability by water and/or acids, a high ion conductivity, stability with respect to reactive species that can be generated at the anode (given for example for perfluorinated polymers), and/or stability in the required pH range, for example with respect to an acid in the salt bridge compartment.

According to certain embodiments, the first ion exchange membrane and/or the first separator are hydrophobic, in particular such that they form a CCM with the electrodes, at least on the side facing the electrodes, so that the reactants of the electrodes are in gaseous form. According to certain embodiments, the anode and/or cathode are at least partially hydrophilic. According to certain embodiments, the first ion exchange membrane and/or the first separator are wettable with water. In order to ensure favorable ion conductivity of ionomers, swelling with water is preferred. In the experiment, it was found that poorly wettable membranes or separators can cause significant deterioration in the ionic binding of the electrodes.

The presence of water is also advantageous for some of the electrochemical conversions at the catalyst electrodes.

-   -   e.g. 3 CO₂+H₂O+2e⁻->CO+2 HCO₃ ⁻     -   e.g. depending on the pH: 2CO₂+2e⁻->CO+CO₃ ²⁻

For this reason, according to certain embodiments, the anode and/or cathode also show sufficient hydrophilicity. Optionally, this can be adjusted by means of hydrophilic additives such as TiO₂, Al₂O₃, or other electrochemically inert metal oxides, etc.

In particular, according to certain embodiments, at least one of the following first separators is used:

-   -   A diaphragm is preferably used if the salt bridge (the         electrolyte in the salt bridge compartment) and the anolyte         comprise or are composed of an identical, preferably inert,         acid, wherein the diaphragm then serves here to keep gases         separated, such that the carbon dioxide does pass into the anode         compartment, and/or if O₂ is produced at the anode, in         particular in order to reduce costs.     -   A cation exchange membrane or a membrane with a cation exchange         layer are used in particular if an electrolyte in the salt         bridge compartment—also referred to in the context of the         invention as a “salt bridge”—and the anolyte are not identical,         and/or in particular if the anolyte contains HCl, HBr and/or HI,         and/or if chlorine production occurs at the anode. As the cation         exchange membrane prevents anions from passing from the anolyte         into the salt bridge, and unlike the diaphragm does not show         open porosity, the anode can be configured more freely. In         principle, in such an embodiment, it is only preferable for the         anode reaction that it does not release any mobile cations other         than protons, which can pass through the CEM into the salt         bridge.     -   A bipolar membrane or bipolar membrane, wherein preferably an         anion exchange layer and/or an anion transport layer of the         bipolar membrane is oriented toward the anode compartment and a         cation exchange layer and/or cation transport layer of the         bipolar membrane is oriented toward the salt bridge compartment,         is used in particular if the salt bridge, i.e. the electrolyte         in the salt bridge compartment, and the anolyte are not         identical, and/or if the anolyte in particular contains bases         and or salts, and/or in the use of aqueous electrolytes. In the         use of bipolar membranes as a first separator in particular, the         anode compartment can be configured independently of the salt         bridge and the cathode compartment, which allows multiple anode         reactions with desired products, and in the use of bases in         particular, cheaper anodes or anode catalysts, for example         nickel- or iron-based anode catalysts for oxygen production, can         also be used.

A bipolar membrane can for example be configured as a sandwich of a CEM and an AEM. In this case, however, the membrane ordinarily comprises at least two layers rather than two membranes placed atop one another. These membranes are virtually impenetrable to both anions and cations. Accordingly, the conductivity of a bipolar membrane is not based on the transport capacity for ions. Instead, the ion transport ordinarily takes place by means of acid-base dissociation of water in the middle of the membrane. In this manner, two oppositely charged charge carriers are generated and transported away by the E field.

As the conductivity of the bipolar membrane is based on the separation of charges in the membrane, however, one must ordinarily expect higher voltage drop.

The advantage of such a structure lies in the decoupling of the electrolyte circuits, because as mentioned above, the bipolar membrane is virtually impenetrable to all ions.

In this manner, a structure can also be realized for a basic anode reaction that obviates the need for constant addition and removal of salts or anode products. This is otherwise possible only with the use of anolytes based on acids with electrochemically inactive anions such as e.g. H₂SO₄. In use of a bipolar membrane, hydroxide electrolytes such as KOH or NaOH can also be used as an anolyte. High pH values thermodynamically favor water oxidation and allow the use of significantly cheaper anode catalysts, e.g. those based on iron-nickel, which would not be stabile in an acidic or neutral environment.

Within the meaning of the invention, therefore, in use of a bipolar membrane as a first separator membrane, the use of bases, e.g. a hydroxide base, as an anolyte is also possible if an acid is used in the salt bridge.

According to the invention, it is not excluded for further membranes and/or diaphragms to be provided in addition to the first ion exchange membrane and the first separator.

According to certain embodiments, the anode comes into contact with the first separator, as described above by way of example. This makes favorable binding to the salt bridge compartment possible. In this case, moreover, no charge transport by the anolyte is needed, and the charge transport path is shortened. Electric shading effects due to supporting structures between the anode and the first separator can therefore also be avoided.

The solid anion exchanger, which is at least partially in contact with the first ion exchange membrane and is contained in the salt bridge compartment, is not particularly limited according to the invention, provided that it is present in the form of a solid—i.e. not in solution—, can exchange anions, and is at least partially in contact with the first ion exchange membrane. Preferably, the solid anion exchanger is hydrophilic.

As discussed above, it is preferable for the solid anion exchanger, at least in the area of the cathode on the opposite side of the first ion exchange membrane, to be substantially in contact therewith—that is, touching it—, i.e. for example for contact with the solid anion exchanger to be more than 50% of the area of the first ion exchange membrane, preferably more than 60%, further preferably more than 70%, in particular more than 80% based on the area of the first ion exchange membrane which is in contact with the cathode.

According to certain embodiments, the solid anion exchanger is not in contact with the first ion exchange membrane over its entire surface, in particular not in the area in which the cathode on the opposite side of the first ion exchange membrane comes into contact therewith, in order to allow fluid transport between the first anion exchange membrane and the solid anion exchanger to be ensured. It is therefore also preferable according to certain embodiments for the solid anion exchanger, at least in the area of the cathode on the opposite side of the first ion exchange membrane, to be in contact therewith, such that contact with the solid anion exchanger is 99% or less of the area of the first ion exchange membrane, preferably 97% or less, further preferably 95% or less, in particular 92% or less, based on the area of the first ion exchange membrane which is in contact with the cathode.

The further mechanical configuration of the solid anion exchanger, which can also be understood as a filling medium, is not particularly limited, and it can be configured for example as a bed of solid anion exchange particles, which are not particularly limited, as a porous structure, for example a spongelike structure, and/or as an e.g. regular porous self-supporting structure. If the solid anion exchanger is in the form of a bed of solid anion exchange particles, the particles should preferably have a particle size of between 5 μm and 2 mm, further preferably between 100 μm and 1 mm, wherein the particle size can be determined for example by sieve analysis. According to certain embodiments, the particles are adapted to the size of the cell and/or to the corresponding flow regime. According to certain embodiments, the solid anion exchanger is present as a bed and/or a porous structure. According to certain embodiments, the solid anion exchanger, optionally with further solid components, e.g. neutral particles or cation exchangers, forms a filling in the salt bridge compartment.

Examples of the mechanical configuration of the solid anion exchanger include: —a compressed bed of e.g. pelletized anion exchange particles; —a spongelike porous structure; —a regular porous self-supporting structure such as can be obtained for example by overmolding of polymer-beads with a solution of the anion exchange material of the solid anion exchanger and subsequent dissolution of the template beads. However, the structure should be at least partially open or completely open in order to allow an electrolyte and gas flow to be ensured.

In addition, porous carrier beads (latex beads) can also be impregnated with an anion exchange ionomer and thus function as ion exchange particles. Here, according to certain embodiments, the bonding of a particle bed of any desired e.g. neutral and/or uncharged particles, such as polymeric particles, to an anion exchange ionomer is preferred. The advantage of this method lies in a greater number of available exchange groups for the transport of anions.

The solid anion exchanger serves as an open extension of the first ion exchange membrane, for example an AEM, into the volume of the salt bridge compartment (e.g. referred to as a gap volume if the salt bridge compartment is configured as a gap). This allows the surface area of anion-conductive components of the electrolysis cell, for example hydrogencarbonate- and/or carbonate-conductive components of the electrolysis cell in CO₂ electrolysis, to be sharply increased. A gas, such as e.g. the CO₂ released by protons from the anode, thus covers a smaller portion of the surface area of the first ion exchange membrane and thus does not cause ionic insulation. Moreover, the solid anion exchanger possesses intrinsic ion conductivity, which can result in the production of an additional conduction path through the salt bridge compartment exclusively by means of solid electrolytes. In the area of the first separator, for example not only HCO₃ ⁻ and/or CO₃ ²⁻, but additionally or solely one anion of the electrolyte used in the salt bridge compartment, for example an acid used, can serve as a mobile charge carrier in the solid ion exchanger. If for example H₂SO₄ diluted in the salt bridge compartment is used as an electrolyte, the solid anion exchanger should preferably be selected such that in addition to favorable HCO₃ ⁻ and/or CO₃ ²⁻ conductivity, it also shows favorable SO₄ ²⁻ conductivity. Accordingly, the material of the solid anion exchanger can be adapted not only to an anion such as HCO₃ ⁻ and/or CO₃ ²⁻ that is cathodically produced, but also to further anions, e.g. in the salt bridge compartment.

The material of the solid anion exchanger is not limited, provided that it is correspondingly adapted to the first ion exchange membrane and/or an electrolyte in the salt bridge compartment, except for the fact that it must be capable of anion exchange and/or anion transport. For example, the solid anion exchanger can comprise an anion exchange resin in which cations are immobilized, preferably alkali metal or alkaline earth metal cations, e.g. by means of complexation, and/or ammonium ions and/or derivatized ammonium ions such as quaternary ammonium ions, further preferably alkali metal cations and/or ammonium ions and/or derivatized ammonium ions such as quaternary ammonium ions. Moreover, for example phosphonium, pyridinium, piperidinium, guanidinium, imidazolium, pyrazolium and/or sulfonium ions can be bound to a carrier of the solid anion exchanger.

According to certain embodiments, the solid anion exchanger comprises in the salt bridge compartment cations, which are immobilized in a polymeric backbone, wherein the cations in particular can exchange hydrogencarbonate ions and/or carbonate ions, wherein these hydrogencarbonate ions and/or carbonate ions should preferably be transportable by the solid anion exchanger in order to provide suitable conductivity in the salt bridge compartment.

Anion exchangers are ordinarily available in solid acidic (e.g. in HSO₄ ⁻ form, where they can also be present as solid acids), neutral (e.g. as a TFO or Cl salt) or basic form (e.g. HCO₃ ⁻ form, weakly basic, or OH⁻ form, strongly basic). In the present invention, depending on the operating method, various such anion exchangers can be present next to one another in a cell according to the invention, wherein a basic anion exchange is preferably used near to and/or on the first ion exchange membrane.

According to certain embodiments, the solid anion exchanger is basic, preferably strongly basic. According to certain embodiments, the immobilized cations of the anion exchanger are configured such that an ion pair formed by them is always present in completely dissociated form, which can be controlled for example by means of pH. According to certain embodiments, the solid anion exchanger comprises hydrogencarbonate, carbonate and/or OH ions and/or anions of the electrolyte used in the electrolyte of the salt bridge compartment, e.g. an acid, as counterions. This allows the transport of hydrogencarbonate and/or carbonate of the first ion exchange membrane to be improved by means of ion hopping (in contrast to the “tunneling” in the Grotthuss mechanism).

Theoretically, it would also generally be possible to correspondingly continue the action of the first ion exchange membrane with a solid, e.g. acidic ion exchanger and a basic liquid electrolyte in the salt bridge compartment. In the special case of CO₂ electrolysis, however, this is not possible because basic solutions are converted by the CO₂ present into neutral carbonate solutions. Within the meaning of the present invention, basic media are also considered to be anion conductors, and acidic media are considered to be proton conductors. As cation transport predominates in neutral (e.g. alkaline) carbonates, a corresponding carbonate solution produced does not correspond to an extension of the first ion exchange membrane into the salt bridge compartment.

According to certain embodiments, the solid anion exchanger is hydrophilic. This means that it is lightly wettable with an aqueous medium, with the result that an aqueous medium such as an aqueous acid can be used in the salt bridge compartment. In addition, the water can also serve as an additional reactant in carbon dioxide reduction, as discussed above, thus allowing favorable conductivity.

In addition, the first anion exchanger and/or a filling comprising the solid anion exchanger, optionally with further solid components, e.g. neutral particles or cation exchangers, also serves to support the separators and/or membranes, e.g. the first ion exchange membrane and the first separator, against one another in the salt bridge compartment. As this filling possesses its own ion conductivity, this type of support does not lead to insulation of electrode areas. In assembling a cell stack, the bed can also be used for force transfer (non-positive locking) via the entire stack.

As discussed above, the filling contains, at least in the area of the cathode/AEM, a solid e.g. strongly basic anion exchanger. The filling can also consist entirely of a solid e.g. strongly basic anion exchanger.

It is advantageous if the chemical nature of the solid anion exchanger is as similar as possible to that of the first ion exchange membrane, e.g. an AEM. Both components can also, for example, be constructed based on the same polymer, wherein, however, e.g. the chain length and/or the degree of crosslinking can be different.

Examples of embodiments in which the salt bridge compartment comprises only a solid anion exchanger are shown schematically in FIGS. 1 to 3. In the figures, the first separator is shown such that it is in contact with the anode. However, this is not absolutely necessary according to the invention, and the separator can also be separate from the anode, such that an anode compartment can also be formed for example between the separator and the anode, and the anode can optionally also comprise on an opposite side of such an anode compartment a compartment for the supply of a substrate, e.g. a gas.

In FIG. 1, the solid anion exchanger 4, for example a strongly basic anion exchanger, is arranged in the salt bridge compartment II, which is located between an anion exchange membrane AEM as a first ion exchange membrane based on an e.g. strongly basic anion exchange material 1 and a cation exchange membrane CEM as a first separator based on an e.g. strongly acidic cation exchange material 3. The cathode K and the cathode compartment I are adjacent to the AEM, and the anode A and the anode compartment III are adjacent to the CEM. As shown in the figure, the solid anion exchanger 4 can be penetrated by fluids such as gases and/or electrolytes. An inlet and an outlet are provided for each of the three compartments I, II, and III.

An alternative embodiment is shown in FIG. 2, wherein the electrolysis cell largely corresponds to that of FIG. 1, except that the CEM has been replaced by a diaphragm D, for example in the form of a hydrophilic gas separator 5. A further alternative embodiment is found in FIG. 3, which also largely corresponds to the embodiment of FIG. 1, wherein the CEM has been replaced by a bipolar membrane BPM in which a cation exchange layer based on an e.g. strongly acidic cation exchange material 3 is oriented toward the salt bridge compartment II, while an anion exchange layer based on an e.g. strongly basic anion exchange material 1 is oriented toward the anode.

In addition to the solid anion exchanger, however, other components of the filling in the salt bridge compartment that can support the release of the hydrogencarbonate taken up in the solid anion exchanger and/or transported therein are also possible.

The filling can thus comprise, e.g. in addition to an e.g. strongly basic and/or weakly basic anion exchanger, nonionic ion exchangers, e.g. polyalcohols, and/or cation exchangers, e.g. weakly and/or strongly acidic cation exchangers, which are not particularly limited. In experiments, for example in the examples of DE 102017211930.6, it was found that less than one equivalent of CO₂ per flowing electron passes through the first ion exchange membrane, e.g. an AEM, into a central salt bridge compartment. This indicates a transfer of CO₃ ²⁻ and/or OH⁻ in addition to HCO₃ ⁻. Acidic additives in the solid anion exchanger, such as cation exchangers, can be used for example for converting CO₃ ²⁻ to HCO₃ ⁻. As a rule, monovalent ions are mobile in ion exchange media, while polyvalent ions are mobile in solution. By means of corresponding addition of this type, an optimum highly conductive conduction path is produced for each type of ion.

FIG. 4 shows a schematic view of an exemplary embodiment in which such a filling is provided in the salt bridge compartment II with a mixed ion exchange material 2 containing an e.g. strongly basic anion exchange material, which for example can be homogeneously mixed. The further configuration of the cell in FIG. 4 corresponds to that of FIG. 1.

A comparison of these two cell designs, with an e.g. strongly basic anion exchange material 4 (FIG. 5) and a mixed ion exchange material 2 containing an e.g. strongly basic anion exchange material (FIG. 6), is also shown in FIGS. 5 and 6 with a generic separator S composed of a generic material 6, which can have a single or multi-layer structure. The further structure corresponds to that shown in FIG. 1.

According to certain embodiments, the solid salt bridge compartment further comprises non-ion-conductive and/or unfunctionalized particulate matter or particles, nonionic ion exchangers and/or cation exchangers, wherein preferably the non-ion-conductive and/or unfunctionalized particles, nonionic ion exchangers and/or cation exchangers, further preferably the non-ion-conductive and/or unfunctionalized particles and/or nonionic ion exchangers, are contained in an area not adjacent to the first ion exchange membrane in an amount of up to 20 vol. %, preferably up to 17 vol. %, further preferably up to 14 vol. %, even further preferably up to 10 vol. % or up to 5 vol. %, based on the total amount of the solid anion exchangers and uncharged particles, nonionic ion exchangers and/or cation exchangers. The mixture of the solid anion exchangers with the uncharged particles, the nonionic ion exchangers and/or the cation exchangers is not particularly limited, and can be homogeneous or heterogeneous, e.g. in the form of layers, etc. The uncharged particles, nonionic ion exchangers and/or cation exchangers are not particularly limited. As the filling is configured in the form of layers, these layers are preferably parallel to the first ion exchange membrane and/or the first separator, wherein the layer adjacent to the first ion exchange membrane comprises the uncharged particles, nonionic ion exchangers and/or cation exchangers in an amount of up to 20 vol. %, preferably up to 17 vol. %, further preferably up to 14 vol. %, even further preferably up to 10 vol. % or up to 5 vol. %, based on the layer, or comprises or contains only the solid anion exchanger.

In contrast, a layer adjacent to the first separator can for example comprise the solid anion exchanger in an amount of up to 20 vol. %, preferably up to 17 vol. %, further preferably up to 14 vol. %, even further preferably up to 10 vol. % or up to 5 vol. %, based on the layer, wherein according to certain embodiments, the remainder can be a solid cation exchanger. A layer adjacent to the first separator can also comprise or contain only the solid cation exchanger. According to certain embodiments, the salt bridge compartment further comprises a solid cation exchanger, which is at least partially in contact with the first separator.

As discussed above with respect to the solid anion exchanger, it is preferable for the solid cation exchanger, at least in the area of the anode on the opposite side of the first separator, to be substantially in contact therewith, i.e. for example in contact with, i.e. touching, more than 50% of the area of the first separator, preferably more than 60%, further preferably more than 70%, and in particular more than 80% based on the area of the first separator, which is in contact with the anode. According to certain embodiments, the solid cation exchanger is not in contact with the first separator over its entire area, in particular not in the area in which the anode on the opposite side of the first separator is in contact therewith, in order to allow fluid transport between the first separator and the solid cation exchanger to be ensured. It is therefore also preferable according to certain embodiments for the solid cation exchanger, at least in the area of the anode on the opposite side of the first separator, to be in contact with 99% or less of the area of the first separator, preferably 97% or less, further preferably 95% or less, in particular 92% or less, based on the area of the first separator, which is in contact with the anode.

It is also applicable for the multi-layer configurations of the salt bridge compartment that solid ion exchangers, which contain no e.g. strongly basic anion exchanger materials and/or are not solid anion exchangers, preferably are not in contact with the first ion exchange membrane, e.g. an AEM, in order to prevent gas release at the contact point.

According to certain embodiments, the composition of the filling, in particular e.g. along the cathode-anode connection line, need not be homogenous. The filling can thus also be coated, for example with an e.g. strongly basic solid anion exchanger or a mixture comprising the solid anion exchanger in the area of the cathode and the first ion exchange membrane, e.g. an AEM, and an e.g. strongly acidic solid cation exchanger or a mixture comprising the solid cation exchanger in the area of the anode and the first separator.

The number of different layers that can be used is not specified, nor is the order thereof, provided that they meet the requirement that the material adjacent to the first ion exchange membrane, e.g. an AEM, contains an e.g. strongly basic anion exchanger.

For example, in such a multi-layer structure, two or more layers of the filling may be present, as shown by way of example in FIGS. 7 to 9 for two layers.

The cell structure with cathode K, anode A, AEM and separator S as well as the cathode compartment I and anode compartment III corresponds to that shown in FIG. 5, and only the structure of the filling in the salt bridge compartment II differs. In FIG. 7, adjacent to the AEM is a layer with an e.g. strongly basic anion exchange material 4, while adjacent to the separator S is a layer with a mixed ion exchange material 2 containing an e.g. strongly basic anion exchange material. In FIG. 8, this mixed ion exchange material 2 containing an e.g. strongly basic anion exchange material of FIG. 7 has been replaced by an e.g. acidic or strongly acidic cation exchange material 3. In FIG. 9, however, in comparison to FIG. 8, the material adjacent to the AEM has been replaced by a mixed ion exchange material 2 containing an e.g. strongly basic anion exchange material.

According to certain embodiments, the filling, even when composed only of the solid anion exchanger, is not closed, such that an amount of an electrolyte and/or a liquid-gas bubble gas can flow through it, i.e. the filling does not comprise any pores or structured free compartments.

A further aspect of the present invention relates to an electrolysis system comprising an electrolysis cell according to the invention. The corresponding embodiments of the electrolysis cell as well as further exemplary components of an electrolysis system according to the invention have already been discussed above and are thus also applicable to the electrolysis system according to the invention. According to certain embodiments, an electrolysis system according to the invention comprises multiple electrolysis cells according to the invention, wherein it is not excluded for other additional electrolysis cells also to be present.

According to certain embodiments, the electrolysis system according to the invention further comprises a return device, which is connected to an outlet of the salt bridge compartment and an inlet of the cathode compartment, which is configured to recycle a reactant of the cathode reaction which can be formed in the salt bridge compartment, back into the cathode compartment.

An electrolysis cell according to the invention is shown by way of example in FIG. 10, wherein the electrolysis cell can be configured with the cathode compartment I, the salt bridge compartment II and the anode compartment III, the anode A, the separator S, the cathode K and the first ion exchange membrane as an AEM, for example according to the structure shown in FIG. 5 or FIG. 6. CO₂ is supplied to the cathode compartment, and the remaining CO₂, product P and optionally water are discharged from the cathode compartment, wherein the water is separated off. CO₂ generated in the salt bridge compartment that may have migrated into the salt bridge compartment can be recycled via a return line to the inlet of the cathode compartment after electrolyte j has been separated from the salt bridge compartment, which can also be recycled. On the anode side, an anolyte A is recycled to the anode compartment III, wherein anodic conversion of H₂O and/or HCl to O₂ and/or Cl₂ is shown here by way of example, wherein the half cell reaction does not limit the invention. The further symbols in FIG. 10 are common fluidic circuit symbols.

According to certain embodiments, the electrolysis system according to the invention further comprises an external device for electrolyte treatment, in particular a device for the removal of dissolved gases from an acid, with the anolyte and/or the electrolyte in particular being treated in the salt bridge compartment, in order for example to remove gases such as CO₂ or O₂ and thus allow recycling of the anolyte and/or the electrolyte to the salt bridge compartment. This is particularly advantageous in cases where both are pumped from a common reservoir, i.e. there is only one common anolyte/electrolyte available for the salt bridge compartment reservoir, which means that the anolyte and the electrolyte in the salt bridge compartment are identical.

According to certain embodiments, the electrolysis system according to the invention comprises two separate circuits for the anolyte and electrolyte in the salt bridge compartment, which can optionally comprise separate devices for electrolyte treatment, in particular devices for the removal of dissolved gases from an acid, or wherein only the circuit for the electrolyte in the salt bridge compartment comprises a corresponding device.

In yet a further aspect, the present invention relates to the use of an electrolysis cell according to the invention or an electrolysis system according to the invention, which can also comprise multiple electrolysis cells according to the invention, for the electrolysis of CO₂ and/or

CO.

In addition, a method is disclosed for the electrolysis of CO₂, wherein an electrolysis cell according to the invention or an electrolysis system according to the invention is used, wherein CO₂ is reduced at the cathode and hydrogencarbonate and/or carbonate generated at the cathode by the first ion exchange membrane migrates to an electrolyte in the salt bridge, wherein the hydrogencarbonate and/or carbonate is also transported through the solid anion exchanger in the salt bridge compartment away from the first ion exchange membrane.

In addition to hydrogencarbonate and/or carbonate, it is not also excluded for formate and/or acetate and/or further generated anions to migrate through the first ion exchange membrane into the electrolyte of the salt bridge compartment. The method according to the invention is carried out with the electrolysis cell according to the invention or the electrolysis system according to the invention. Accordingly, all of the features discussed with respect to the electrolysis cell according to the invention and the electrolysis system according to the invention are also applicable to the method according to the invention. In particular, the cathode compartment, the cathode, the first ion exchange membrane, the anode compartment, the anode, the separator, the salt bridge compartment and the solid anion exchanger, as well as further components, have already been discussed with respect to the electrolysis cell according to the invention and the electrolysis system according to the invention. Therefore, the corresponding features can thus be implemented correspondingly in the method according to the invention. Conversely, the method according to the invention can also be implemented with the electrolysis cell according to the invention or the electrolysis system according to the invention, so that comments or aspects with respect to the method for the electrolysis of CO₂ according to the invention can also be applied thereto, for example with respect to an electrolyte in the salt bridge compartment and accompanying configurations of the components of the electrolysis cell, such as e.g. the first separator.

With the methods according to the invention, CO₂ is electrolyzed, wherein, however, it is not excluded, in addition to CO₂, for a further reactant such as CO to also be present on the cathode side, which can also be electrolyzed, i.e. for a mixture to be present that comprises CO₂, as well as e.g. CO. For example, a reactant on the cathode side comprises at least 20 vol. % of CO₂, e.g. at least 50 or at least 70 vol. % of CO₂, and in particular, the reactant on the cathode side can comprise up to 100 vol. % of CO₂. In principle, the electrolysis cell according to the invention can also convert pure CO, wherein in this case, of course, no CO₂ is then released in the salt bridge compartment.

In the method according to the invention, an electrolyte, i.e. a liquid medium, flows through the filling comprising the solid anion exchanger or composed of the solid anion exchanger. The electrolyte is not particularly limited, but according to certain embodiments may also be aqueous. According to certain embodiments, the salt bridge compartment thus comprises an aqueous electrolyte. It may correspond to the anolyte and/or catholyte, as appropriate, or may be different therefrom.

According to certain embodiments, the electrolyte of the salt bridge compartment comprises an acid, preferably a water-soluble or water-miscible acid. According to certain embodiments, the electrolyte contains at least 10⁻⁶ mol/l of H⁺ and/or hydrated variants thereof, preferably at least 10⁻⁴ mol/l, further preferably at least 10⁻³ mol/l, and even further preferably at least 10⁻² mol/l. According to certain embodiments, the electrolyte of the salt bridge compartment comprises essentially no mobile cations other than H⁺ and/or hydrated variants thereof. Preferably, according to certain embodiments, the electrolyte comprises no mobile cations other than protons, with the exception of mobile cations in a number of common contaminants. The electrolyte serves to discharge the CO₂ and keep the filling moist.

The at least one acid in the electrolyte in the salt bridge compartment is not particularly limited, but is preferably a water-soluble and/or water-miscible acid, such as for example HCl, HBr, HI, H₂SO₄, H₃PO₄, HTfO (trifluoromethane sulfonic acid), etc. The use of at least one acid in the electrolyte promotes the CO₂ release from hydrogencarbonate and/or carbonate in the solid anion exchanger, for example a basic or strongly basic ion exchanger. The release of the CO₂ preferably takes place in the volume of the salt bridge compartment and not at the contact surface between the filling comprising the solid anion exchanger or the solid anion exchanger and the separator, as this would also lead to considerable voltage losses.

According to certain embodiments, an improvement in gas release in the salt bridge compartment can be achieved using multi-layer fillings, as described above. Of course, it is also possible to build up an electrolyte gradient in the salt bridge compartment in order to achieve a preferred release in the salt bridge compartment and not on separators such as the first ion exchange membrane, the first separator and optionally further contained separators and/or ion exchange membranes, for example with multiple electrolyte inlets to the salt bridge compartment or layers of fillings, wherein in this case laminar flows are also optionally possible in order to produce such electrolyte gradients.

According to certain embodiments, the filling comprising the solid anion exchanger or composed of the solid anion exchanger is preferably ion-conductive in order to improve charge transport by the electrolyte.

It is not excluded in the method according to the invention to use only water as an electrolyte in the salt bridge compartment. For this purpose, however, the first separator is preferably configured as an ion exchange membrane comprising a cation exchanger, for example as a cation exchange membrane (CEM), or as a bipolar membrane (BPM). Preferably, in addition to the solid anion exchanger, the solid filling also contains acidic components, e.g. cation exchangers. Because of the conductivity, however, the use of an acidic solution is preferred.

If the first separator is configured as a diaphragm, the electrolyte comprises in the salt bridge compartment at least one acid, as the diaphragm is not intrinsically ion-conductive. The electrolyte of the salt bridge compartment can for example correspond to the anolyte, but can also be different therefrom.

A particularly preferred embodiment of the method according to the invention lies in the use of the solid anion exchanger, optionally in a mixture with further components in the filling of the salt bridge compartment, in combination with an acidic electrolyte. In this manner, compared to the prior art, the contact surface between the anion exchanger of the first ion exchange membrane and the acidic media can be sharply increased. In solutions according to the prior art, e.g. in US 2017037522 A1 and DE 102017208610.6, the surface area of the first ion exchange membrane is also the transition to the acidic medium in all cases. According to the invention, this transition is moved into the volume of the salt bridge compartment, thus massively increasing the surface area. As a result, the insulating action of the gas bubbles generated in CO₂ electrolysis less adversely affects the cell voltage. The action of the anion exchanger/transporter contained in the first ion exchange membrane as a transporter for anions can be continued by the filling in the salt bridge compartment.

According to certain embodiments, the anode compartment comprises an anolyte, which comprises a liquid and/or a dissolved acid, preferably wherein the anolyte and/or the acid in the salt bridge compartment or the electrolyte in the salt bridge compartment comprise no mobile cations other than protons and/or deuterons, in particular no metal cations. According to certain embodiments, an acid in the salt bridge compartment comprises no mobile cations other than protons and/or deuterons, in particular no metal cations. According to certain embodiments, the anolyte comprises no mobile cations other than protons and/or deuterons, in particular no metal cations. Mobile cations are cations that are not bound by a chemical bond to a carrier and/or in particular have an ion mobility of more than 1.10⁻⁸ m²/(s·V), in particular more than 1.10⁻¹° m²/(s·V). According to certain embodiments, during the anodic half reaction, no mobile cations other than “D⁺” and H⁺″, in particular no metal cations, are released or produced. In such a case, therefore, for the special case of O₂ generation at the anode, for example, water (in particular in the case of a CCM anode) or acids with non-oxidizable anions may be used as an anolyte or reagent. Accordingly, for halogenation at the anode, particularly in this case, the halogen-hydrogen acids HCl, HBr and/or HI are suitable, wherein for example halide salts are not suitable in use of a diaphragm as a first separator membrane, but can be used in use of a bipolar membrane as a first separator membrane. The use of SO₂ in the anolyte for the production of sulfuric acid or H₂O for the production of H₂O₂, etc. is also.

An exemplary method according to the invention will now be discussed with respect to a special configuration of the electrolysis cell of FIG. 5, which in the following will be further implemented. The comments refer to a special configuration of the electrolysis cell of FIG. 5, such that the following explanations do not limit the embodiment shown in FIG. 5, which is described above. In such an exemplary method, the cathode compartment I of the salt bridge compartment II is separated by a composite of a CO₂ reducing cathode K and an AEM. CO₂, for example moistened CO₂, flows through the cathode compartment I, where it is reduced for example to CO and C₂H₄. The moistened CO₂ flow constitutes the substrate supply to the cathode. It then constitutes the catholyte within the meaning of a classical three-compartment cell.

The salt bridge compartment II is separated from the anode compartment III by the first separator S (e.g. a diaphragm, a bipolar membrane, a cation-conductive membrane) in conjunction with the anode A, wherein—as discussed above—it is also possible for the anode compartment III to be directly adjacent to the first separator. The salt bridge compartment II is packed with a solid filling through which substances can flow that contains an e.g. strongly basic anion exchanger, and is flowed through by an electrolyte flow, which in addition to water can also comprise an acid.

The first separator can be freely selected e.g. from a cation exchange membrane (CEM), a not intrinsically ion-conductive hydrophilic gas separator (diaphragm), or a bipolar membrane (BPM), in which the anion-conductive layer is preferably oriented toward the anode. In use of a diaphragm, the electrolyte in anode compartment III and the liquid electrolyte in the salt bridge compartment II are preferably identical and conductive.

The anolyte, e.g. aqueous HCl, aqueous H₂SO₄, H₂O, etc., flows through the anode compartment III, which can provide the anode A with substrate. In cases where the selected electrolyte of the salt bridge compartment and the anolyte are identical, they can also be obtained from a common reservoir, wherein in particular, however, suitable devices are present in order to prevent the discharge of dissolved gases (degassing), e.g. in a return line of the electrolyte.

As mentioned above, in a departure from FIG. 5, the anode compartment III can also be located between the anode A and the first separator S. In such a case, however, the anolyte must be conductive.

The cathode compartment I and the anode compartment III can additionally comprise e.g. electrically conductive, e.g. non-closed, structures, which are used for contacting of the electrodes. If the anode is not adjacent to the first separator, the requirement of conductivity can be dispensed with. Preferably, the anolyte contains only salts and thus mobile “non-H⁺” cations if the first separator is a bipolar membrane.

The electrochemical conversion at the anode is not further limited, wherein it preferably leads to the transition of H⁺ from (bipolar membrane) or through (diaphragm or CEM) the first separator into the electrolyte of the salt bridge compartment.

Provided this is useful, the embodiments, configurations and improvements above can be combined with one another as desired. Further possible configurations, improvements and implementations of the invention comprise combinations of features of the invention described above or below in connection with the examples, even if they are not explicitly mentioned. In particular, the person skilled in the art may also add individual aspects as improvements on or supplements to the respective basic form of the present invention. 

1. An electrolysis cell, comprising: a cathode compartment comprising a cathode; a first ion exchange membrane, which contains an anion exchanger and which is adjacent to the cathode compartment, wherein the cathode comes into contact with the first ion exchange membrane; an anode compartment comprising an anode; and a first separator, which is adjacent to the anode compartment; a salt bridge compartment, wherein the salt bridge compartment is arranged between the first ion exchange membrane and the first separator, wherein the salt bridge compartment comprises a solid anion exchanger, which is at least partially in contact with the first ion exchange membrane.
 2. The electrolysis cell as claimed in claim 1, wherein the solid anion exchanger comprises in the salt bridge compartment cations, which are immobilized in a polymeric backbone.
 3. The electrolysis cell as claimed in claim 1, wherein the solid anion exchanger is present as a bed and/or a porous structure.
 4. The electrolysis cell as claimed in claim 1, wherein the solid salt bridge compartment further comprises uncharged particles, nonionic ion exchangers and/or cation exchangers.
 5. The electrolysis cell as claimed in claim 1, wherein the first separator is a cation exchange membrane, a bipolar membrane or a diaphragm.
 6. The electrolysis cell as claimed in claim 1, wherein the solid anion exchanger is basic, or strongly basic.
 7. The electrolysis cell as claimed in claim 1, wherein the solid anion exchanger is hydrophilic.
 8. The electrolysis cell as claimed in claim 1, wherein the salt bridge compartment further comprises a solid cation exchanger, which is at least partially in contact with the first separator.
 9. An electrolysis system, comprising: an electrolysis cell as claimed in claim
 1. 10. The electrolysis system as claimed in claim 9, further comprising: a return device that is connected to an outlet of the salt bridge compartment and an inlet of the cathode compartment, which is configured to recycle a reactant of the cathode reaction, which can be formed in the salt bridge compartment, back into the cathode compartment.
 11. A method for the electrolysis of CO₂ with an electrolysis cell as claimed in claim 1, the method comprising: reducing CO₂ at the cathode, wherein hydrogencarbonate and/or carbonate generated at the cathode by the first ion exchange membrane migrates to an electrolyte in the salt bridge compartment, wherein the hydrogencarbonate and/or carbonate is also transported through the solid anion exchanger in the salt bridge compartment away from the first ion exchange membrane.
 12. The method as claimed in claim 11, wherein the salt bridge compartment comprises an aqueous electrolyte.
 13. The method as claimed in claim 11, wherein the electrolyte of the salt bridge compartment comprises an acid, or a water-soluble acid, or a water-miscible acid.
 14. The method as claimed in claim 11, wherein the electrolyte of the salt bridge compartment essentially comprises no mobile cations other than H⁺ and/or hydrated variants thereof.
 15. A method for the electrolysis of CO₂ and/or CO, the method comprising: performing electrolysis using the electrolysis cell as claimed in claim
 1. 16. The electrolysis cell as claimed in claim 2, wherein the cations exchange hydrogencarbonate and/or carbonate ions.
 17. The electrolysis cell as claimed in claim 4, wherein the uncharged particles, nonionic ion exchangers and/or cation exchangers are contained in an area adjacent to the first ion exchange membrane in an amount of up to 20 vol. %, based on the total amount of the solid anion exchanger and the uncharged particles, nonionic ion exchangers and/or cation exchangers.
 18. The electrolysis cell as claimed in claim 4, wherein the uncharged particles and/or nonionic ion exchangers are contained in an area adjacent to the first ion exchange membrane in an amount of up to 20 vol. %, based on the total amount of the solid anion exchanger and the uncharged particles, nonionic ion exchangers and/or cation exchangers. 